doc: `require` behavior on case-insensitive systems

This adds a paragraph in the Module Caching Caveats section about the
behavior of require when Node is running on top of a file system (e.g.
HFS) or operating system (e.g. Windows) that will not consider the case
of file paths to find files.

Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Alexis Campailla <orangemocha@nodejs.org>
Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Roman Reiss <me@silverwind.io>